Amplification control



March 2, 1937. N. P. cAsE 2,072,708

AMPLIFICATION CONTROL Y Filed Aug. 4, 1934 3 SheetS-Sheet l INVENTOR Nfl50N FiF/7) (,455

ATTO R N EYS N. rP.4 CASE' AMPLIFICATION COISTROL 3 Sheets-Sheet 2 FiledAug. 4, `19:54

INVENTOR NELSON PERRY CASE @wmd ATTORNEYS Mamh 2, 1937.

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AMPLIFICATION GOTROL Filed Aug. 4, 1934 3 Sheets-$heet 3 www a6 52 64ree 25p 512 m21. 20411111196819216318# FREU/VCY INVENTOR /VES/V Pff/FfC462'- BY 79m, MJ

ATTORNEYS Patented Mar. 2, 1937 Unirse STAT-ss Parri-:NTA OFFICEAMPIJFIQATION CONTROL Nelson Perry Case, Bayside, N. Y., assigner toHazeltine Corporation, Jersey Gity, N, J., a corporation ofDelawareapplication August 4, 1.934, serial No. '138,494

18 Ulaixns. (Cl. 179-171) 'Ihis invention relates to ampliner systemsand more particularly to means for automatically varying the relativeamplication of voltages at diierent frequencies within the frequencyhand in a desired manner and as a function of the signal level at aselected point in the system.

In accordance with the general principles of this invention, signalvoltage is applied to the input of an ampliiier system which includesone portion designedto amplify voltages within a substantially broadband of frequencies and another portion designed to amplify voltageswithin a narrow frequency band which may be within or adjacent to thewide band of frequencies. A

`i portion of the amplified signal voltage is rectied and applied to thecontrol element of a vacuum tube which alters the transmissionproperties of that portion of the amplifier designed to amplify Withinthe narrow frequency band, thus autoor matically varying theoutput-vs.-frequency characteristic of the ampliiier system as a whole,depending upon the strength of signal voltage at a given point thereof.For illustration, this invention will be described 25 vin detail asapplied to an audio-frequency amplifying system, such as might beemployed in a radio broadcast receiver or in a phonograph, a1- thoughthe same general principles may be applied to a system adapted toamplify radio-fre- A 30 quency currents or combined radioandaudiofrequency currents. By means of the4 invention an audio-frequencyamplier may be so arranged as to increase the ratio of low-frequencyvoltages' to voltages of higher frequency in the output, 35 whenever theoutput voltage level is reduced below a designated value.

It is a-wellknown characteristic of the human ear that it becomesrelatively insensitive to sounds of very low frequency when the soundintensity o falls to a low level. The effect of this characteristic,-inrelation to music, is as follows:

When a musical program, as from 'a radio broadcasting station, is beingreproduced at an ,acoustic level nearly the same as that at the 5microphone, ,the reproduction should sound most natural if theoutput--vs.frequency characteristie of the electro-acoustic system isat, i. e., if all the component frequencies are present in thereproduction in the same relative amplitude as in 50 the originalrendition.

Due to the small size of the averageroom in which a radio receiver orphonograph is operated, it is not normally desirable to have the repro--duced sound as loud as the original. However, 55 when me vo1ume levelis reduced, toa satisfactorily low value without altering the relativeamplitudes of the various frequency components, it

is found that, because of the above-mentioned characteristic of the ear,the music sounds-thin, j as though there were a serious deciency in the5 reproduction of the low frequencies or bass notes. Therefore, in orderto produce the psychological effect of true tone balance at low acousticlevels, it is necessary to introduce frequency discrimination in theampliiier so that the actual low frequency response is much greater thanthat for high frequencies. I

An object of the present invention is, therefore, to provide a new4 andimproved sound reproducing system 4in which the output volume 15 may bereduced to any audible level without an apparent loss or diminution ofthe low frequencies. Attempts have been made t'o achieve the desiredresult by connecting appropriate circuit eleacteristic of the ampliersystem varies in shape according to the setting of the manualaudio-frequency volume level control. While this method results in anappreciable improvement over the performance of an uncompensatedamplifier, it is not entirely satisfactory, since the actual output ofthe amplier Variesl over a wide range with a vfixed setting of thevolumecontrol device, because of the normal variation of musical amplitudebetween pianssimo and fortissimo, or for other reasons.

It is, therefore, an additional object of this invention to augment thelow-frequency response of an amplifier system relative to the responseat high frequencies whenever the output level is low, regardless ofWhether the level is low because of a low input level or because thevolume level control device is set at a low value.

In one form which this invention may take, the audio-frequency voltagesare amplied by an audio-frequency amplifier system including asupplementary low-frequency amplier. This e supplementary amplifiercontains a frequencymentary amplifier in such a manner that whenever theoutput voltage of the amplifier is large, the degree of amplification orgain of the supplementary amplifier will be reduced and the audio- 5frequency voltages will be amplified only in the main amplifier.Whenever the output voltage decreases to a sufficiently low level, thesupplementary amplifier will operate to amplify the low audiofrequencies disproportionately. l0

controlled by the integrated values of the variousaudio-frequency'voltages that are present in the output circuit,although it is not necessary, and

is often not desirable, to integrate uniformly over the entireaudio-frequency band.. It is usually.

preferable to give the higherv frequency components relatively greaterweight in controlling the gain of the supplementary amplier.

be controlled by the usual manual volume control arrangement providedinthe input of the special amplifier constructed in accordance with.`this invention. In a'radio receiver, the usual automatic radio-frequencyamplication control and manual volume control may be provided, operativeupon the signals before they are supplied to the amplifier systemconstituting the present invention. y 1

An alternative arrangement incorporating this .invention may be providedwherein the-amplification characteristic of an amplifier stage is causedto be selective in favor of the frequencies which it is desired toaccentuate whenever the output voltage of the amplifier is low.

ing the present `invention may be provided wherein a portion of thevoltages ofv medium and high audio frequency is bypassed and thusprevented 'from reaching the final amplifier, whenever the f 40 totaloutput voltage is low.' In this arrangement,

- Whenever the output voltagefalls to a sumciently low level, for anyreason whatever, the low-:fre-l Fig. 5 is a circuit diagram of an`audio-frequency amplifier constructed in accordance with a modificationof the present invention;

d0 Fig. 6 is a circuit diagram of another modification ofthe presentinvention; and

Fig. 7 is a diagram showing thev percentage re' lationships, for highand 'low output, voltage levels, between the various frequencies in thec5 audio-frequency output of an amplifier constructed in accordance withthis invention. A

Referring first to Fig. l, the ordinates represent the sound pressureand the abscissas represent audio frequencies. 'Ihe curve, which wastaken 70 from Speech and Hearing by Fletcherygives the minimum audiblevalue of the sound pressure for various audio frequencies. It can beseen from this figure that the average ear is most sensitive tofrequencies lying between 1000 and 4000 cycles 75 and that the soundpressure necessary for audi- The action `of the supplementary amplifieris l The output volume of the reproducer or loud speaker may VAnother'alternative arrangement incorporatao'zavoe bility at a frequencybetween 64 and 128 cycles is many times that necessary for audibility at1000 cycles.

Fig. 2 shows a schematic diagram of an amplifier system constructed inaccordance with a preferred embodiment of this invention. In this figurethe signal to be amplified is supplied to the input amplifier il and thesupplementary amplifier I2. The outputs of both the amplifiers II and I2are supplied to the output amplifier I3 by' which the signals arefurther amplified. A portion of the output of the output amplifier issupplied to the frequency filter, rectifier, and time constant circuitindicated at` I4. Here the voltages of the various frequencies areintegrated, the higher frequencies usually being favored over the lower,and rectified to produce a controlling bias for theA supplementaryamplifier I2. The circuitv of It includes a time constant circuit inorder that instantaneous variations of the output level will not affect`the 'supplementary amplifier. The operation of this system will bedescribed hereinafter in connection with the description Y of Fig. 4.

Fig. 3 isa schematic diagram of an alternative arrangement of anamplifier system embodying this invention. In this figure the signal issupplied to the input amplifier II. The signal is then passed to theselective amplification control I5 and thence to the output amplier I3.A portion of the output of the amplifier I3 is supplied to the frequencyfilter, rectifier and time constant .circuit I4. Here the voltages ofvarious frequeninput terminals I-2 are connected across a' voltagedivided P1, the variable tap of whichserves to impress a desired portionof the input Vvoltage across the input of the amplifier tube V1. Theinput signal is amplified in tube V1 and is supplied through the usualnetwork including the coupling condenser C1 to the grid of the tube V2.lThe signal is further amplified in the amplifier tube V2 and impressedthrough an appropriate network including the coupling condenser C2 totheinput of the amplier tube V4. Here the signal is further amplified andimpressed through the transformer T1 upon the inputs of the push-pullconnected vacuum tubes V5, V5. The output of the push-pull connectedamplifier is connected to the primary of output transformer T2. Acrossone-half of the primary o f the output transformer T2 is connected avoltage divider Pz by means of the variable contact of which a desiredportion of the output voltage is supplied through the coupling condenserC3 to the diode plates of the tube V2.

A circuit including resistor- Rz, condenser C4 and condenser C5 isconnected between the plate of the tube V1 and the cathode thereof. Aconnection between the condensers C4 and Cs is made to the input grid ofthe supplementary amplifier tube V3 by which low-frequency-signals areimpressed upon the tube Va` in a manner A diode resistor R1 is connectedbe-. tween ythe diode plates and the cathode of the hereinafter morefully described. The rectified output terminals z resistance of R1.

voltage developed across the resistor R1 and the normal voltage`acrossthe cathode resistor 'Ra of the tube V2 determine thel bias voltage ofthe vacuum tube Vs in a manner which will be described more fullyhereinafter. The low-frequency voltages are amplified in the tube Va andsupplied through an appropriate network including the condenser Cs tothe grid of the amplier tube V4 'in which they are amplified togetherwith the signalvoltages supplied to the grid of tube V4 throughcondenser C2.

Appropriate resistors, condensers and voltage supply sources areprovided as indicated. Any

of the tube V3. The gridv of the tube Va is therefore excited by thevoltage drop across the condenser Cs. It is obvious that at highfrequencies only a small percentage of the voltage appears across thiscondenser, while at very low frequencies most of the voltage isdeveloped across it. The resistor lR2 is chosen to be of high enoughresistance so that the condenser Cs does not affect the input to thetube V2 appreciably at high frequencies. Tube V3 is a high gainamplierftube, and the circuit constants are so 'chosen that with normalbias on the grid of the tube Vs, the low Vfrequencies which areamplified by V3 are amplifled-to several times the extent to which thesignals are amplified by the tube V2 and its associated circuitelements. The signals which have been amplified by tubes V2 and Va arecombined and applied to the grid of the tube V4 which feeds thepush-pull output tubes V5 ln the usual manner. Usually a loud speaker isconnected to the The signal voltage taken from the voltage divider P2 isfed through the condenser C3 to the diode detector embodied in the tubeV2. The diode leak resistance R1 must be high enough so that it producesa negligible loading effect upon the output circuit. The value of thecondenser C3 must be carefully chosen in relation to the If thiscondenser is too small, the bass notes will be accentuated too much whenthe output level is high, providing the music being reproduced containsfew treble notes. On

the other hand, if this condenser is too large, the

desired accentuation of the bass at low` output levels cannot beachieved because the bass notes themselves will excite the diode circuittoo much, on account of the relatively greater amplitude of the lowfrequencies in most music.

At high output levels, a large negative bias voltage is built up at thediode plates of the tube V2, and this voltage is transmitted through thefilter network Re-Cv-R'z to the grid of the tube V3, suiilcient torender the tube V3 inoperative. Under these conditions the amplifier hasa normal, flat frequency characteristic.

As the output voltage decreases, from any cause whatever, the negativevoltage on g the diode plates decreases until, at `a certain outputlevel,

75 the tube V3 commences to be operative. As already noted, the inputcircuit of me tube v1 favors the lower frequencies with reference to`the higher. The output circuit of the tube Va the plate circuitimpedance is practically thatl of its plate resistor R5 in parallel withthe grid leak R of the tube V4; for very high frequencies, the platelresistor R4 of the tube V1 is also effectively in parallel with theresistances R5 and Re, and since R4 is considerably lower in resistance,the gain through the tube V3 for high frequencies is considerably lowerthan for low frequencies. Therefore when Va is operative, it serves toaccentuate the bass notes relative to those of the middle and trebleresisters.

As the output voltage continues to decrease, the potential on the diodeplates of tube V2 approaches that of the cathode of V2, and under thiscondition, the tube Vs operates with maximum gain. Therefore, for alloutputs below a certain level, the bass receives the full accentuationof which the system is capable. The choice of the level at which toatten out the gaincurve l and the choice of the particularfrequency'range to accentuate may be preselected by a variation of thecircuit elements involved.

Although anyvappropriate amplier tubes may be used, the followingAmerican type tubes are suitable:

V1 The triode section of a type 85 tube V2 A type 85 Va A type 77 V4andV Vs-- type 42 tubes connected as triodes.

Although the resistors and condensers shown may have any appropriatecharacteristics, the following values are given for the purpose ofillustration;

The alternative system of Fig. 5 includes an amplifier foramplifying.the high and low frequency signals to a greater extent thanthe mid dleregister signals whenever the output volume is reduced. Inthis circuit, where circuit elements having functions corresponding tothose of similar-elements in Fig. 4 are similarly designated, the inputterminals I--2 are connected across the voltage divider P1, the variablecontact of which serves to impress any desired portion of the inputvoltage across the input of the tube V1. The output of the tube V1 isconnected through an appropriate network including the condenser C1 tothe control grid of the amplifier tube V4. The output of the tube V4 isconnected through the transformer T1 to the inputsof the push-pullconnected amplifier tubes V5, Vs. The

to the primary oftransformer T2, the secondary of which is connected tothe output terminals 3 4 across whicha loud speaker may be con 5 nected.

A voltage divider P2 is connected across onehalf of the primary of theoutput transformer T2. The variable contactof voltage divider P2 isiconnectecl through condenser C3 to the diode plates vof the tube Va. Aresistor R1 is connected between the cathode of the tube V3v and the'diode plates. 'I'he screen-grid of the tube V1 is 'connected through anetworkv comprising the parallel impedances L, R, and C tothe cathode of15 the tube Va. Appropriate biasing resistors, coupling resistors andvoltage supply sources are provided as indicated. Specific details ofthese elements are not essential to the operation of the invention. Forinstance, any appropriate volt- 20 age source may be provided.

In operation, whenever the voltage of the output at the terminals 3 ishigh, a high voltage is impressed upon the diode plates of the tube V3vresulting in a considerable direct-current volt- 25 age being built upacross the resistor R1. 'I'his causes the grid of tube V3 to have a highnegative bias so that this tube acts as ahigh resistance inthe screencircuit of the tube V1. If the resistance of the tube V3 is largecompared with 30 the anti-resonant impedance of the circuit L R-C, thenthe amplifier will have Aa fiat characteristic. For low level signals,the resistance of the tube V3 is relativelysmall and therefore thegain-. -vs.-frequency characteristic curve of the tube V1 will have adipat theresonant frequency of L-C. The characteristics of L- RC are chosento cause the combination to be broadly resonant at the middle registerby means vof which the amplification of the amplifier at middle registerfrequencies may be greatly reduced at low output volumes. The circuitL--R-C may be replaced by a more complicated network to produce anydesired shape of response curve.

Attention is now invited to Fig. 6, which is a circuit diagram of anarrangement related to the block diagram of Fig. 3. The signal voltageis applied to the input terminals i and 2, across which is connected avvoltagedivider P1. The portion of the input voltage appearing betweenterminal 2 and the variable tap on the voltage divider is applied to thecontrol grid of the high gain' amplifier tube V1. tion of the action ofthe filter circuit L-Ca-Cs for the moment, it will beseen that amplied55 signal voltage appears across the coupling reslstor R9 and istransferredl through the coupding condenser C1 to the grid yoi theamplifier tube V4; After Abeing further amplified by tube V4, the signalvoltage is applied through the push- 70 is applied through the couplingcondenser C3 to the diode plates of tube V3. Resistance R1 serves as aload resistance for the diode circuit of V3. 'I'he rectified current owsthroughRi resulting in a pulsating direct-current voltage appearing 75across R1. The alternating current components Neglecting considerameansof the filter circuitR'r-e-C'z-Ra'andfthe resulting direct-currentvoltage ap'pliedas-"ay bias voltage to the grid'oi tube V21.-y Tube vV3`'acts as a variable resistance rconnected in the coirifv mon groundlead of-the low-'pass filter network Direct-current plate`vvrpotential@@iisV supplied to V3 through theimpedanceZ-,Ywhich -ofVthis voltage `are subtantially removed .byl

has a high impedance to all frequencies' lvin the useful range andplaysl no partir! lthe'action of the device except to prevent signalcurrents from passing directly from the plate of tube Va to groundthrough the plate voltage source, in this case indicated by a batteryfor the sake of simplicity.

The action of the control circuit is as follows:

When the signal level across the primary of T2 is high, a relativelyhigh negative bias voltage is applied to the grid of V3, which causes V3to have a very high resistance to signal currents. Consequently thefilter circuit Cs--Ir-Cs has a `negligible effect on the transmissionproperties amplifier is altered so that low frequencies are amplified toa greater `extent than high frequencies.

In each of the circuits shown, the resistors R7 and Ra and the condenserC7 comprise a filter to prevent the alternating current components fromreaching the grid of the control tube and Y to introduce a time constantto average the controlling voltage overv a desired, time interval.

In-the example given the time constant is 1A; second. This time constantis not critical, but should be small enough to enable the bias to followthe normal variations in loudness of a musical passage, and large enoughso that no noticeable controlling action appearsduring the duration of asingle normal musical note.

Fig. 7 illustrates lthe output-vs.`-frequency characteristic of anampliiier constructed in accordance with the circuit arrangement of Fig.4. The curves A and B represent, respectively, the output-vs ;-frequencycharacteristic at various audio frequencies for low and high outputlevels, respectively. The ordinates represent the output voltage .at-thevarious frequencies with reference to the output at 400 cycles. It canbe seen that with low output voltage the low frequencies between 64 and128 cycles predominateby from about 300 to 800% relative to thefrequencies of more than 500 cycles. Therefore as the volume of `theoutput is reduced by any means whatever, the low-frequencyamplification.

becomes relatively greater. Thus with this arrangement, the response at.the low frequencies is maintained atva suficiently high level to remainabove the threshold value of the average ear in order that the lowfrequencies will not seem to fade out as the output level is'reduced.

Whereas this invention has been described for the purpose ofillustration with reference to the three specific examples given above,it is to be understood that any appropriate circuit in which the shapeof the outputvs.frequency characteristic curve is controlled by avoltage derived from the rectification of all or part of theaudio-frequency components of the signal being amplified may be used toaccomplish the objects of this invention.

What I claim:

1. In a multi-stage vacuum tube amplifier system adapted to amplifysignal voltages within a wide frequency band, a path including mainamplifier means adapted to amplify voltages of all of the frequencies ofsaid band and a second. path connected in circuit with at least aportion of said first path and including supplementary "audio-frequencyamplification which includes` amplifier means adapted to amplifyvoltages of only a portion of the frequencies within said band, meansfor combining the voltages so amplifled, and means for automaticallyvarying the amplifying properties of the supplementary amplier means inaccordance with the amplitude of signal voltages.

2. In a multi-stage vacuum tube amplifier System adapted to amplifysignal voltages within a wide frequency band, a .path including mainamplifier means adapted to amplify voltages of all of the frequencies ofsaid band and a second path connected in circuit with at least a portionof said rst path and including supplementary amplifier means adapted toamplify voltages of only the lower frequencies within said band, meansfor combining the voltages so amplified, and means for automaticallyvarying the amplifying properties of the supplementary amplifier meansin accordance with the amplitude of signal voltages.

3. In a multi-stage vacuum tube amplifier system adapted to amplifysignal voltages within a wide band of frequencies, a path including mainamplifier means adapted to amplify voltages of all of the frequencies ofsaid band, a second path connected in circuit with at least a portion ofsaid first path and including supplementary amplier means adapted toamplify only voltages within a narrow frequency range in said wide bandof frequencies, a restifier associated with said system to develop adirect-current potential from signal current supplied thereto, andconnections for applying said direct-current potential to a controlelement of said supplementaryamplifler means to control automaticallythe amplification of said supplementary 'amplifier means.

4. In an amplifier system, the method of audiofrequency amplificationwhich includes auto.

substantially uniformly amplifying voltages of all of the frequencies ofa wide band, simultaneously supplementally amplifying the voltages ofthe lower frequencies of said band and automatically increasing saidsupplemental amplication of said low-frequency voltages as theintegrated audio-frequency voltage level at a given point in theamplifier decreases.

' 6. In an amplier system for amplifying slmultaneously voltagesrepresentative of a broad band of audio-frequencies, said voltages beingsubject to variations in amplitude, an input circuit, an amplidercoupled to said input circuit,

an output circuit, means for integrating the voltage level at a givenpoint in the amplifier system with discrimination against a portion ofthe audio-frequency band, means for averaging band in accordance withthe variations of said integrated and averaged voltage level.

7. In an amplifier system for amplifying simultaneously voltagesrepresentative of a broad band of audio-frequencies, said voltages beingsubject to variations in amplitude, an input circuit, an amplier coupledto said input circuit, an output circuit,` means for integrating thevoltage level at a given point in the amplifier with discriminationagainst a portion of the audio-frequency band, means for averaging saidintegrated voltage level over a preselected interval of time, and meansfor automatically increasing the ratio of the amplication oflowfrequency voltages to the amplification of voltages of higherfrequency as the said integrated and averaged voltage level decreases.

8. In an amplifier system for amplifying simultaneously voltagesrepresentative of a broad band of audio-frequencies, said voltages beingsubject to variations in amplitude, an input circuit, an amplifiercoupled to said input circuit, a frequency-selective networkincorporated in said system, a variable-resistance device controllingthe action of said frequency-selective network, an output circuit, meansfor integrating the voltage level at a given point in the ampliervsystem with discrimination against a portion of the 4audio-frequencyband, means for averaging said integrated voltage level over apreselected interval of time, and means for automatically varying theresistance of said variable-resistance device in accordance with thevariation of said integrated and averaged voltage level.

9. In an amplier system for amplifying simultaneously voltages ofvarying amplitudes representative of a broad band of audio-frequencies,an input circuit, an amplier, an output circuit, a. supplementaryamplier system connected in parallel with at least a portion of saidamplifier, a frequency-selective network incorporated in saidsupplementary amplifier system, means for integrating the voltage levelat a point of said amplifier with discrimination against a. portion ofthe audio-frequency band, means for averaging said integrated voltagelevel over a preselected interval of time, and means for varying theamplication of the supplementary amplifier in accordance with thevariation. of said integrated and averaged voltage level.

10. In an amplifier system for amplifying simultaneously-voltagesrepresentative of a broad band of audio frequencies, said voltages beingsubject to variations in amplitude, an input circuit, an amplifiercoupled to said input circuit, an. output circuit, means for integratingthe voltage level at a point of the amplier, comprising a rectifier, acoupling between said p'oint of said amplifier and said rectiiler, saidcoupling including a frequency-selective network, and a load resistancefor said rectiier across which a controlling voltage is developed; andmeans for averaging over a preselected time interval the integratedvoltage developed across said rectier load resistance, comprising acoupling circuit betweeny the high-potential end of said load resistanceand the point at which said contro series resistance together with shuntcapacitance, said resistance and said capacitance being so proportionedthat the time constant of said coupling circuit is small enough toenable the controlling voltage to vary substantially 'as the amplitudeenvelope of a normal musical passage and large enough so that thecontrolling voltage will not vary substantially during a single .musicalnote.

1l. .in an audio-frequency amplifier system including an input and anoutput, the method of audio-frequency amplication which includessubstantially uniformly amplifying voltages representative of the entireaudio-frequency band in one path, in a separate path amplifying voltagesrepresentative of the .low-frequency portion of the band, and increasingsaid last mentioned amplincation as the-level of the voltage in saidoutput is decreased.

12. In an amplier system for amplifying voltages of varying amplitudeswithin a broad band of frequencies, an input amplifier, an outputamplier. a path coupling said input amplier `to said output amplifier, aseparate path coupling said input amplifier to said output amplifier andincluding a supplementary amplifier, and a nlter whereby only apredetermined part of the frequencies of said band are impressed uponsaid supplementary amplifier, and means for automatically controllingthe amplification in said supplementary amplifier in accordance with'the amplitude of the amplified voltage in the output of said outputamplifier.

13. In an amplifier system for amplifying voltages covering a broad band`of frequencies and subject to variation in voltage level, an input, anamplifier tube connected to said input, a supplementary amplifier, acoupling separately connecting said input andsaid supplementaryamplifier, said coupling including means for discriminating againstvoltages of certainfrequencies in said band of frequencies, individualoutput circuits coupled to said amplifiers, means for controlling theamplification of said .supplementary ampliiier in accordance with theamplitude of the signal voltages, and means introducing a time constantinto the action of said amplification controlling means.

14. 'In an amplier system for amplifying Voltaojaavo's voltage isapplied, said coupling circuit including agesv covering a broad band` offrequencies and subject to variation in voltage level, an input circuit,an output circuit, two parallel amplifier circuitscoupled betweensaidinput and output circuitsa network included in one of saidv amplifiercircuits for discriminating against voltages of certain frequencies insaid band of frequencies, and a coupling between the output circuit andsaid last-mentioned amplifier circuit for'accent- -uating therein theamplification of voltages of 16. An amplifier system for amplifyingsignal voltages within a Wide frequency band, a main path designedsubstantially uniformly to pass the frequencies of said band, a separatepath electrically connected in. circuit with at least a portion of themain path and designed to pass only a portion of the frequencies of saidband, and means for automatically controlling the passage of frequenciesthrough said separate path according to the amplitude of said signalvoltages.

17. In a multi-stage vacuum tube amplifier system, a path includingamplier means designed substantially uniformly to amplify signalvoltagesof all frequencies within a wide band, a separate pathelectrically connected in circuit with at least a part of the iirstsaidpath, including ampliner means adapted to amplify voltages offrequencies within a narrow range included substantially in said wide-band, and means for automatically varying the amplification of saidsecond amplifier means according to the amplitude of said signalvoltages.

18. In an amplifier system, the method of amplifying voltages offrequencies within' a wide band. which includes simultaneously andautomatically augmenting the amplification of lowfrequency voltages asthe amplitude of the signal decreases and maintaining the amplificationof higher frequency voltages substantially constant.

NELSON PERRY CASE.

